U.S. Pat. No. 4,054,967 assigned to Formax, Inc. discloses a method and apparatus for moulding food patties, in particular hamburger patties moulded from ground meat. In moulding the food patties, a supply of the food product from which the patties are to be formed is maintained in a supply hopper. From the hopper the food product is fed to a food pump of the reciprocating plunger type. It is mentioned that one of the problems with conventional food patty moulding techniques results from “churning” of the food product by a positive feed mechanism that supplies the food product to the pump. This results from the positive feeding action used to force the food product into the pump intake, and the attendant difficulty in avoiding circulatory flow. This churning may cause separation of the food product, e.g. separation of fat from other tissue in ground meat. In addition, the churning effect of the feeder mechanism may grind the food product into smaller particles than desired. In both instances, an undesired deterioration of the food product may result. Therefore, the food pump according to this document has a large access port for introducing food product into a narrow pump chamber. A supply of food product is continuously maintained in a position completely blocking the large access port that leads to the pump chamber so that the access port is prevented from being open to atmosphere. To introduce food product into the pump chamber, the pump plunger is rapidly withdrawn from the chamber, past the access port. The rapid withdrawal of the plunger from the chamber produces a partial vacuum and draws a quantity of food product from the supply hopper through the access port into the chamber. Thus, the food product is not forced into the food pump using a positive feeding action. Instead, the food product is drawn into the pump, primarily by the vacuum developed on rapid withdrawal of the plunger from the pump. Although this reduces churning of the food product, a positive feed mechanism (feed screws) in the supply hopper is still needed because the food product has to be continuously maintained in a position completely blocking the large access port that leads to the pump chamber. The operation of the feed screws still result in churning of the food product with resultant deterioration.
It has been acknowledged in U.S. Pat. No. 4,182,003 also assigned to Formax, Inc. that the vacuum intake pump of the kind described in U.S. Pat. No. 4,054,967 reduces the churning problem, but may not eliminate it entirely. Therefore, U.S. Pat. No. 4,182,003 proposes to align two counter-rotating feed screws along axes parallel to the direction of movement of the pump plunger. This enables the feed screws to function much in the manner of a gate valve with respect to the intake opening for pump chamber. The feed screws advance the food product into a feed screw end housing, which leads directly to the intake of the food pump. During the pumping stroke of the plunger, reverse movement of food product out of the pump chamber is blocked by the feed screws, that are now stationary, and by the food product contained in the feed screw end housing. The blocking effect of the feed screws and the food product in the feed screw end housing is such that there is little tendency for reverse pumping of food product from the pump chamber back through the pump intake opening. Instead, the food product is sheared along the level of the top of the plunger and is compressed in the forward end of the pump chamber. It is described that the gate valve action of the feed screws with respect to the food pump reduces churning of the food product at the pump intake. However, the operation of the feed screws, even if they are run only intermittently, causes agitation of the food product which induces deterioration of the food product. This affects the quality of the moulded food patties.